1. Field of the Invention
The present invention relates to a laminate, and more particularly, to a polyolefin-coated paper using a base paper having incorporated therein or coated thereon a specific copolymer. The base paper after incorporation of or coating of the copolymer and prior to application of the polyolefin may be subjected to a corona discharge treatment.
2. Description of the Prior Art
Proposals have been heretofore made to coat the surface of a paper with polyolefins such as polyethylene, polypropylene or the like for use in various fields, e.g., as milk cartons, wallpapers, food wrapping papers, poster papers, printing papers, garbage bags, etc., to render the paper hydrophobic.
One conventional method of coating a paper with a polyolefin is the so-called dry lamination which comprises uniting the paper and a polyolefin film with an adhesive. Another method which is more efficient is an extrusion coating method which comprises extruding a polyolefin melt down onto a moving paper film through a T-die of an extruder, and, after adhering the polyolefin to the paper film, followed by uniting both under pressure while cooling the assembly at the same time.
However, polyolefins are non-polar high molecular weight materials and as a result good adhesion to paper is not necessarily achieved. Furthermore, from an economical viewpoint, a reduction in the amount of polyolefin coated and an increase in coating speed have been extensively required. From a quality consideration, a reduction in the extrusion temperature employed has also been required still more. Therefore, depending upon the material or the process of production thereof the tendency is to obtain a paper with insufficient adhesion between the paper and the polyolefin layer. It is recognized in the art that various problems in practical use are encountered with a coated paper which has insufficient adhesion.
That is, when a coated paper having insufficient adhesion between the paper and the polyolefin layer is cut and finish-worked, stretched portions of polyolefin layer which are beard-like or tufty tend to occur at the cut surface thereof, which results in a bad appearance at the cut section. When such occurs to an extreme, the polyolefin is not completely cut off so that two or more sheets still remain connected. Further, polyolefin tends to be peeled off from paper at the cut areas due to friction or the like.
An example of the application of a polyolefin coated paper to special fields includes the so-called water proof photographic printing paper. A water proof photographic printing paper is generally obtained by coating a light-sensitive photographic emulsion layer on a support comprising paper, both surfaces of which has been coated with polyolefins, for the purpose of enhancing the efficiency of the development processing by reducing the development processing period of time for the photographic printing paper.
In this case, if the adhesion between the polyolefin and the paper is insufficient, a peeling-off of the polyolefin from the paper occurs at the edge portions thereof, particularly when such is wet during the development processing, and a development processing solution is absorbed in the paper at the peeled portions so that the rapid processing desired can not be accomplished, in addition to the marked decrease in commercial value due to the bad appearance resulting from the inferior cut ends upon cutting and finish-working as mentioned above. At worst, the polyolefin layer may completely be peeled from the paper. Therefore, it is extremely essential in a polyolefin coated paper that sufficient adhesion between the paper and polyolefin be obtained.
The simplest method for providing good adhesion between a paper and a polyolefin comprises increasing the polyolefin resin temperature to a high temperature of about 320.degree. C. or higher upon extrusion coating. However, this temperature is a temperature which is very close to the temperature at which the polyolefin is decomposed, and a part of the polyolefin may have already been decomposed. The decomposed polyolefin comes into contact with air on passing from the extruder through a T-die before being coated on paper, is oxidized and discolors, which results in a deterioration of the color hue of the coated paper or a bad smell. These are fatal defects particularly when such is contemplated for use as a food wrapping. In addition, when such is to be used as a photographic support, thermal decomposition products adversely affect photographic emulsions causing fogging, which is also fatal for this use. Further, the so-called fish eyes are undesirably formed during extrusion coating over a long period of time so that the working efficiency is decreased and a decrease in the commercial value of the coated paper and an inferior appearance result.
It is also possible to improve the adhesion between a paper and a polyolefin by increasing the thickness of the polyolefin coating. However, this increases the cost and further changes the curl balance which causes difficulty in handling.
In addition, activation treatments such as subjecting a paper surface or the like to a corona discharge treatment as described in U.S. Pat. No. 3,411,908, British Pat. No. 1,005,631 and Japanese Patent Publication No. 22834/73 are also conventional. However, in general, a marked improvement in adhesion strength is not obtained.
For the purpose of providing sufficient adhesion strength, a method which comprises previously coating a material having good adhesion both to a paper and to a polyolefin onto a paper surface prior to coating the polyolefin on the paper (a so-called primer treatment) and then performing an extrusion coating has been proposed. Examples of materials which are employed for such a primer treatment include polyethylene imines, low molecular weight polyethylene emulsions, ethylene-sodium acrylate copolymers, vinyl acetate resins, polyacrylic acid esters, polyvinylidene chloride, polyacrylic acids and the like. However, these materials have the disadvantages that adhesion is sufficient and in particular, when the resin temperature is decreased upon extrusion coating, the adhesion suddenly decreases.
As a result of extensive investigations to eliminate the above disadvantages, a method of improving adhesion between a paper and a polyolefin, in particular, upon extrusion coating at low temperatures, has now been found.